The present invention relates to a discharge lamp especially suited for forward lighting application of a vehicle such as an automobile, truck, bus, van or tractor. More particularly, the discharge lamp is a metal halide type which is particularly suited for a vehicle such as an automotive and has means for reducing the typically expected losses occurring during the operation of a metal halide lamp.
Automotive designers are interested in lowering the hood line of cars in order to improve their appearance and also their aerodynamic performance. As discussed in the cross-reference U.S. application Ser. No. 157,359, the amount that the hood lines may be lowered is limited by the dimensions of the automotive headlamp, which, in turn, is limited by the dimensions of the light source which is typically comprised of a tungsten filament.
U.S. applications having Ser. Nos. 157,359, and 157,436, respectively disclose a xenon lamp and a xenon-metal halide discharge lamp having dimensions which are substantially reduced relative to a tungsten light source, which, in turn, allow for the reduction of the overall size of the reflector of the automotive headlamp housing a light source so that the hood line of the automobile may be substantially reduced by the automotive designers. In addition to the xenon lamp and xenon-metal halide lamps, it is desired to provide a metal halide lamp for automotive applications so as to allow for aerodynamic styling of automobiles. Further, it is desired to provide for a xenon-metal halide lamp having improvements related to automotive and other applications. Still further, in addition to the metal halide light source serving the needs of automobile, it is desired that an improved metal halide light source find lighting applications in the home, office and other commercial and industrial usages.
In one lighting application particularly suited for automobiles, it is desired to provide a metal halide lamp that may be operated from a low frequency alternating current (A.C.) power source or direct current (D.C.) power source. In such A.C. and D.C. applications, the metal halide lamp typically experiences the effects of catephoresis which cause the halides of the metal halide lamp to be moved or swept into the end regions of the lamp so as not to contribute to providing the desired illumination of such lamp. It is desired that means be provided which substantially reduce or even eliminate the detrimental effect of catephoresis on the operation of the metal halide lamp.
A second disadvantage typically related to metal halide lamps, particularly metal halide lamps having relatively small dimensions so as to be adapted to automotive applications, is that these lamps typically include a sodium iodide as a part of their fill, and the sodium ions of this ingredient may migrate by electrolysis through the fused silica of the metal halide lamp during operation. As the sodium is lost and the free iodine of the sodium iodide is left behind in the lamp, the lamp illumination deteriorates through the loss of sodium radiation. The free iodine causes the operating voltage of such lamp to begin to rise which may ultimately cause the metal halide lamp to experience a failure. It is desired that means be provided to substantially reduce or eliminate the sodium ion migration problem typically associated with the operation of metal halide lamps.
A third disadvantage related to metal halide lamps, is concerned with the structure necessary for mounting the metal halide light source within an outer envelope so as to form the overall lamp. The structure, in particular a metal structure, when subjected to incident radiation emitted from the metal halide light source commonly causes the metal structural members to emit photoelectrons. Some of these photoelectrons drift to the outer surface of the metal halide light source, charging such a surface in a negative direction and accelerating the electrolysis of the sodium ions through the fused silica of the metal halide lamp. It is desired to minimize or reduce the metal structural members for mounting the metal halide light within its related lamp so as to correspondingly reduce the electrolysis of the sodium ions through the fused silica created by metal structural members which emit photoelectrons.
A further disadvantage related to metal halide lamps, is the disadvantageous feature created by the presence of hydrogen and water which may diffuse out of the metal halide lamp. It is desired that means be provided to reduce the detrimental effects of hydrogen and water without contributing to any further disadvantageous operation of the metal halide lamp such as the creation of photoelectrons that would otherwise cause the loss of the sodium ion from the metal halide lamp.
A still further disadvantage that may possibly occur with a halide lamp is related to the rupturing of the metal halide lamp that is typically operated at a relatively high pressure. Upon the limited possibility of such an occurrence, the high pressure within the metal halide lamp may cause the material of such a metal halide lamp to be dislodged at a relatively high velocity which may possibly fracture the outer envelope in which the metal halide lamp is housed. It is desired that confinement means be provided so as to reduce the possible effects of the rupturing of such a metal halide lamp operated at a relatively high pressure.
Accordingly, it is an object of the present invention to provide a metal halide lamp having means so as to reduce the detrimental effects of catephoresis typically created by low frequency A.C. operation or D.C. operation of such a lamp.
It is a further object of the present invention to provide means to reduce the sodium ion migration typically experienced for a metal halide lamp.
It is a further object of the present invention to reduce the sodium ion migration caused by metallic mounting members emitting photoelectrons which contribute to the loss of the sodium ions of the metal halide lamp.
It is still a further object of the present invention to provide containment means so as to reduce the possible detrimental effects that may occur during the unlikely event of the rupturing of the metal halide lamp operated at a relatively high pressure.